History of Forest Ecology 391 



stages and that there are but a few dimax or ultimate formations. 

 Sen ft (6) followed the succession of vegetation on bare xero- 

 phytic slopes. He noted in successive stages, lichens, mosses, 

 grasses, herbs, shrubs, woody thickets, and lastly forest on the 

 same area. Warming (7) contributed largely to this field of in- 

 vestigation in bringing together and summarizing the many 

 results up to his time. He considers the changes of vegetation 

 under three heads, namely, whose occurring on newly formed soils 

 or upon surfaces exposed for the first time ; those due to slow 

 changes upon soils with a vegetative cover ; and those occurring 

 without accompanying changes in climate and soil. From the 

 standpoint of succession he establishes three kinds of associations : 

 initial, transitional and climax. 



The more recent investigations in plant succession have en- 

 deavored to follow very closely the diflFerent stages in sucession, 

 and if possible to ascertain the physical factors which determine 

 succession. Cowles (41) has shown that the forest succession 

 on the sand dunes of Lake Michigan consists principally of asso- 

 ciations dominated by cottonwood, pine, black oak, white and 

 red oaks, and beech and maple, in the order named. These repre- 

 sent a continuous series extending from the pioneer xerophytic 

 trees to the mesophytic climax forest of the region. Fuller (42) 

 defined "growth water" as the soil moisture in excess of wilting 

 coefficient, since he believed that none of the water absorbed 

 from the soil whose moisture content is below the wilting 

 coefficient is used for the growth of the organism. Later the 

 same writer (43) based a study in succession upon the ratio of 

 evaporation to growth water. This ratio for the beech-maple 

 forest, oak-hickory forest, oak dune, pine dune, and cottonwood 

 dune associations were shown to have comparative values of 

 100, 65, 200, 17, and 15 respectively and differences thus indi- 

 cated are sufficient to be efficient factors in causing succession 

 Roberts (44) finds the order of succession in the Holyoke range 

 as follows: bare soil, herbs, cedar, birch, pine, oak-hickory, chest- 

 nut, and the beech-maple-hemlock forest. In a most interesting 

 manner the author shows by diagram that should fire destroy 

 any of these stages the series must begin all over again; but if 

 one of these associations is logged off it will usually revert to 

 the preceding one before again returning to the original form. 

 This is a most significant result of forest fires in the succession 



